1. Field of the Invention
The present invention relates to a spectrum spreading communication system, and more particularly to a spectrum spreading communication system suitable for a random burst signal transmitted from a plurality of mobile stations to a radio base station on a same frequency band at a same time using a same spreading code.
2. Description of the Related Art
As a conventional mobile communication system using a spectrum spreading communication system of this type is known the technique disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 5-227124), for example. FIG. 1 is a block diagram illustrating an example of a reception system used in a CDMA communication system as the conventional technique.
The CDMA communication system is composed of a single radio base station 300 and a plurality of mobile stations 200. A spreading code is allocated for the communication between the radio base station 300 and each mobile station 200 such that the bidirectional communication can be performed. The radio base station 300 synthesizes a pilot signal spread based on a short period code P, one period of which is set as one bit, and data transfer signals spread based on long period codes A, C and E corresponding to the respective mobile stations 200. The synthesized signal is transmitted to the mobile stations 200. The respective mobile stations 200 performs inverse spreading to the reception data transfer signal using the long period codes A, C and E allocated to the mobile stations, respectively. Also, the respective mobile stations 200 spreads data transfer signals using the transmission long period codes B, D and F, respectively. The radio base station 300 performs inverse spreading to a reception data transfer signal using reception long period codes B, D and F corresponding to the respective mobile stations 200.
In such a structure, synchronization is established between a generation timing of the short period code P and each of the long period codes A to F. Also, a time mark is contained in a pilot signal to indicate a bit position of the long period code corresponding to the pilot signal so that it is possible to separate the long period codes into groups and to allocate them to a lot of mobile stations. Therefore, the synchronization can be quickly established using the pilot signal of the short period code. Also, the synchronization of a corresponding group part of the long periods code can be established based on the time mark contained in the pilot signal so that the synchronization of the long period code can be easily and quickly established. Therefore, it is possible to provide the CDMA communication system in which it is made possible to allocate unique codes to a lot of mobile stations while keeping an access random function.
However, in such a technique disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 5-227124), special controls are required. That is, there are synchronization establishment between the pilot signal and the data communication signal, insertion and excerption of the time mark, and the generation of a partial mark which matches to the time mark. Therefore, there is a problem in that the circuit scale becomes large so that the circuit becomes complicated.
Also, in another conventional technique, a plurality of mobile terminals transmits an same spreading code. In case of the reception of burst signals in which the plurality of mobile terminals perform access requests to a radio base station at the same time using the common spreading code, the respective signals interfere with each other.
Also, in such burst communication, it is difficult to strictly perform the transmission power control which is indispensable technique in the spectrum spreading communication system.
For these reasons, the dispersion of the levels of the signals received by the radio base station becomes large so that a delay profile used to detect paths is disordered. As a result, there is a problem in degradation of a path detecting capability.
Further, even if an initial synchronization correlating unit (a searcher) captures burst signals from the mobile stations, the mobile station in each path can not be easily identified. Therefore, there is a problem in that it is impossible to optimally realize the rake synthesis which is the technique to remarkably improve the communication quality peculiar to the spectrum spreading communication system.
In addition to the above reference, a spectrum spreading communication system is disclosed in Japanese laid Open Patent application (JP-A-Showa 56-102143). In this reference, a base band signal is formed by adding a data to be transmitted and a pilot signal. On a side of reception, the pilot signal is detected such that synchronization is established.
Also, a transmission signal system is disclosed in Japanese laid Open Patent application (JP-A-Showa 59-50603). In the this reference, a sync signal is transmitted with a transmission signal via a digital phase modulation communication line. Upon reception, the control of an antenna is performed using the sync signal as an its identification reference signal.
Also, an automobile telephone system is disclosed in Japanese laid Open Patent application (JP-A-Showa 63-202144). In the this reference, a plurality of communication channels are divided in frequency. A spectrum spreading communication is used for a line control channel to perform line control of the communication channels. A spectrum spreading code sequence on the line control channel corresponds to a mobile station telephone number.
Also, a receiving apparatus in a spectrum spreading communication system is disclosed in Japanese Examined Patent application (JP-B-Heisei 2-39139). In the this reference, the receiving apparatus is composed of a first quasi-noise code generating section, a correlation detector, a clock generating section, a second quasi-noise code generating section, a control section, and a demodulating section. The first quasi-noise code generating section generates a first quasi-noise code which always changes the phase for an inputted spectrum spread reception signal. The correlation detector always detects the correlation state between the spectrum spread reception signal and the first quasi-noise code to detect the state of the spectrum spread reception signal. The clock generating section generates a clock to control the first quasi-noise code generating section based on the detecting result by the correlation detector such that the phase of the first quasi-noise code always changes for the spectrum spread reception signal. The second quasi-noise code generating section generates a second quasi-noise code. The control section controls the second quasi-noise code from the second quasi-noise code generating section in accordance with the detecting information by the correlation detector such that the spectrum spread reception signal is coincident with the second quasi-noise code. The demodulating section performs the demodulation of the spectrum spread reception signal based on the second quasi-noise code controlled by the control section.
Also, a receiving apparatus in a spectrum spreading communication system is disclosed in Japanese Laid Open Patent application (JP-A-Heisei 6-152564). In the this reference, [step 111] existence or non-existence of a carrier of a Busy Tone is detected prior to a packet transmission. When an xe2x80x9cLxe2x80x9d level is detected over a predetermined time, a step 112 is executed. Also, a signal outputted from its station and a signal outputted from another station are compared to know a channel acquirement result of its station. When the signal is changed to the xe2x80x9cHxe2x80x9d level during a channel acquirement period, the channel acquirement is discarded and then a receiving process is performed. [Step 112] A shift register outputs its identification code as the Busy Tone for one data. [Steps 113 and 114] It is determined whether all the Busy Tone data are transmitted, a PN code is continuously transmitted based on a PN code table, when a signal corresponding to a B station comparison level is kept in the state of xe2x80x9cLxe2x80x9d level for the channel acquirement period. Thus, a frequency band can be effectively used and a plurality of communication stations can communicate with each other at the same time in a spectrum spreading communication system.
Also, a spectrum spreading communication system is disclosed in Japanese Laid Open Patent application (JP-A-Heisei 7-30514). In the this reference, a matching filter 1 performs an inverse spectrum spreading to a reception signal R1 to which a spectrum spreading is performed, with an inverse spreading code R2 to produce a pulse sequence R2. A transmission path estimating section 2 estimates the transmission characteristics of a multi-path transmission path in response to a pilot signal R1a contained the reception signal R2 to produce tap coefficients R4 (R4a). A transversal filter 3 performs tap weighting using the tap coefficients R4 to produce the maximum ratio synthesis signal R5 of the pulse sequence R3. A multi-path interference reproducing section 5 reproduces a multi-path interference signal R8 in response to the tap coefficients R4a, a demodulating signal R6 and the inverse spreading code R2. A subtracting section 7 subtracts an interference signal from a delay synthesis signal R5a which has been delayed from a synthesis signal R5 by a delay section 6. A determining section 8 determines a subtracting signal R9 to output a demodulation signally. Thus, a multi-path interference signal of a received spectrum spreading signal is removed in a base band.
Also, a CDMA/TDD system radio communication system is disclosed in Japanese Laid Open Patent application (JP-A-Heisei 7-221700). In the this reference, in addition to the structure of a conventional CDMA/TDD system radio communication system, a base station is further composed of means 15 for generating a pilot signal having a constant transmission power level to each of the mobile stations and known to the mobile station, and means 16 for transmitting the pilot signal to the mobile station via a transmission path. Also, each mobile station is further composed of means 19 for measuring transmission power of the received pilot signal and means 8 for controlling transmission power of a power amplifier circuit 10 based on the reception power of the measured pilot signal.
Also, a CDMA/TDD system radio communication system is disclosed in Japanese Laid Open Patent application (JP-A-Heisei 7-226710). In this reference, in addition to the structure of a conventional CDMA/TDD system radio communication system, a base station is further composed of means 15 for generating a pilot signal having a constant transmission power level to each of the mobile stations and known to the mobile station, means 16 for transmitting the pilot signal to the mobile station via a transmission path, and single tone generating means 26 for always transmitting to the mobile station, a single tone having a single frequency and a constant transmission power level. Also, each mobile station is further composed of means 19 for measuring transmission power of the received pilot signal, means 28 for measuring a reception power of the single tone, and means 8 for controlling transmission power of a power amplifier circuit 10 based on the measured reception power of the pilot signal and the measured reception power of the single tone.
Also, a rake receiving apparatus of a direct spectrum spreading communication system is disclosed in Japanese Laid Open Patent application (JP-A-Heisei 7-231278). In this reference, a direct spreading modulation signal S30 of N paths is received by a first to N-th inverse spreading sections 501 to 50N and then the received signals S30 are subjected to inverse spreading and demodulation using a first to N-th spreading sequences synchronous with N signals S30 in which reception timings are different from each other. The demodulation data D401 to D40N are synthesized by a synthesizing section 51. The level differences between a signal having the maximum one of levels of the N signals S30 and other signals are determined. When each of the level differences is larger than a predetermined threshold value T, inverse spreading means for performing the inverse spreading and demodulation to the signals having level difference larger than the threshold value T from the maximum level (for example, 50N-2, 50N-1, 50N) is controlled to be set to an off state.
The present invention is accomplished to solve the above-mentioned problems. Therefore, an object the present invention is to provide a spectrum spreading communication system in which communication can be performed in good quality, even if a plurality of mobile stations access a radio base station with burst signals using a same spreading code and a same frequency band.
Another object of the present invention is to provide a spectrum spreading communication system in which a multi-path can be identified to make a rake synthesis possible so that communication quality can be improved.
In order to achieve an aspect of the present invention, a spectrum spreading communication system including a radio base station includes a receiving section, a correlation detecting section, a rake synthesizing section and a control section. The receiving section receives a radio signal to produce a digital burst signal. Transmission data of the radio signal respectively include different pilot patterns and data, are subjected to spectrum spreading using a single spreading code, and are transmitted on a same frequency band. The correlation detecting section performs a tracking operation to the burst signal for each of paths to produce a tracking resultant data. The paths are specified based on a path data. The correlation detecting section also detects a component of the burst signal correlated with the spreading code for each path based on the tracking resultant data. The rake synthesizing section performs rake synthesis to the components from the correlation detecting section for each of the pilot patterns. The control section determines path candidates from the burst signal to output the path data corresponding to the path candidates to the correlation detecting section, and controls the rake synthesizing section to perform the rake synthesis to the components from the correlation detecting section for each of the pilot patterns.
Each of the transmission data includes an identifier data of a mobile station which has transmitted the transmission data. The radio base station further includes a data decoding section for decoding signals outputted from the rake synthesizing section, and for extracting the mobile station identifier data contained in the decoded signals to specify mobile stations, respectively.
The control section may include a plurality of searcher section respectively provided for the pilot patterns, wherein each of the plurality of searcher section determines larger likelihood levels between the corresponding pilot pattern and the burst signal, and a path control section for determining the path candidates based on the larger likelihood levels over the pilot patterns to output the path data to the correlation detecting section, and for controlling the rake synthesizing section to perform the rake synthesis to the components from the correlation detecting section for each of the pilot patterns. In this case, each of the searcher section performs in-phase addition between a code sequence of the corresponding pilot pattern and a pilot pattern component of the burst signal to determine the path candidates having larger results of the in-phase additions.
Also, the control section may include a a searcher section for determines the path candidates having larger likelihood levels between the spreading code and the burst signal, a plurality of kind detecting sections respectively provided for the pilot patterns, wherein each of the plurality of kind detecting sections determines one of the pilot patterns for each of the path candidates, and a path control section for outputting the path data to the correlation detecting section based on the detected pilot patterns by the plurality of kind detecting sections, and for controlling the rake synthesizing section to perform the rake synthesis to the components from the correlation detecting section for each of the pilot patterns. In this case, each of plurality of kind detecting sections performs in-phase addition between a code sequence of the corresponding pilot pattern and a pilot pattern component of the burst signal to determine the path candidates having larger results of the in-phase additions.
Also, the correlation detecting section may include a plurality of tracking sections, each of which performs a tracking operation to the burst signal for a corresponding one of the paths to produce a portion of the tracking resultant data, and a plurality of correlating sections which are provided for the plurality of tracking sections, and each of which detects a component of the burst signal correlated with the spreading code for each path based on the tracking resultant data.
In addition, a spreading code decoding section may include the correlation detecting section, the rake synthesizing section, the control section and the data decoding section. In this case, the spectrum spreading communication system further comprises a plurality of the spreading code decoding sections respectively provided for a plurality of the spreading codes.
In order to achieve another aspect of the present invention, a method of communicating from the plurality of mobile stations to the radio base station, in a spectrum spreading communication system including a radio base station and a plurality of mobile stations, comprising the steps of:
transmitting transmission data from the plurality of mobile stations as radio signals on a same frequency band, wherein the transmission data respectively include different pilot patterns and data, the different pilot patterns are allocated to the plurality of mobile stations, and the transmission data are subjected to spectrum spreading by the plurality of mobile stations using a single spreading code, respectively;
receiving the radio signals to produce a digital burst signal;
determining path candidates from the burst signal to generate the path data corresponding to the path candidates;
performing a tracking operation to the burst signal for each of paths to produce a tracking resultant data, the paths being specified based on a path data;
detecting a component of the burst signal correlated with the spreading code for each path based on the tracking resultant data; and
performing rake synthesis to the components from the correlation detecting section for each of the pilot patterns to produce rake synthesis signals for communication.